Protective coating for magnesium and zinc



United States Patent 3,122,457 PROTECTIVE CQATENG FQR MAGNESIUM AND ZINCHerbert K. De Long and James A. Brown, both oi Midland, Mich, assignorsto The Dow Chemical Company,

Midland, Mich, a corporation of Delaware No Drawing. Filed Sept. 18,1961, Ser. No. 138,620

6 Claims. (Cl. 148-621) This invention relates to an improved coatingfor magnesium and zinc articles and to a method for producing such acoating. It more particularly is concerned with a surface conversioncoating for magnesium and Zinc which renders these metals resistant tocorrosive attack.

The term magnesium as used herein refers to magnesium metal or amagnesium-base alloy which contains about 85 percent or more magnesium.The term zinc as used herein refers to zinc metal or a Zinc-base alloywhich contains about 85 percent or more Zinc. The termc-hromate-manganate coating as used herein refers to a coating which isformed by subjecting magnesium and zinc articles to the action of asolution containing a chromium compound and a manganese compound in anacidic aqueous solution containing sulfate ions and which is thought tocontain chromate ions (CIO and manganate ions (MnOg') as constituents.Such a coating imparts corrosion-resistant properties to these articles.

Many conventional coating solutions and coatings formed on magnesium andzinc by immersion in, or brushapplication of, aqueous solutionscontaining chromium compounds have been unsatisfactory in a number ofparticulars. Variations in the color of the coated article, lack ofuniformity of thickness of the coating, formation of powdery material onthe coating during the coating process, failure to build up a coating ofdesired thickness, and high metal loss are some of the disadvantagesencountered in the use of known coating solutions. Furthermore, in manyof the processes for forming chromate coatings the accumulation ofaltered chromium and magnesium or zinc compounds in the bath duringusage ne e sitates discarding and replacing the bath at uneconomicallyfrequent intervals.

It is a principal object of the present invention to provide an improvedchromate based surface conversion coating which can be used on bothmagnesium and zinc. It is another object of the present invention toprovide a method for producing a surface conversion coating of uniformcolor. A further object of the present invention is to provide achromate coating solution which is economical of active ingredients andhas a long service life. A still further object is to provide a solutionfor chromate coating which undergoes very little build up of undesirablecompounds and which maybe revived by mere addition of active ingredientsto the solution and therefore need not be discarded after relativelyshort periods of use. It is an additional object of the presentinvention to provide a novel chromate-manganate undercoating formagnesium and zinc articles and particularly pressure die-cast articleswhich is an excellent base for paint coatings. These and other objectsand advantages will become apparent from the detailed descriptionpresented hereinafter.

This invention is based upon the discovery that specific acidic aqueouschromate-manganate solutions in the presence of sulfate will produce asurface conversion coating on magnesium and zinc base articles whichcoating has a desirable uniformity of color and thickness and highresistance to corrosion.

The aqueous acidic chromate-manganate bath of the present inventioncomp-rises on a weight basis from about 2 to about percent andpreferably from about 12 to about 18 percent chromate expressed as Na CrO -2H O, from about 0.05 to about 1 and preferably from about 0.3

to about 0.6 percent manganate expressed as KMnO from about 0.2 to about0.55 percent and preferably about 0.36 to about 0.45 percent sulfate,expressed as 98 percent H and will have a pH of from about 0.9 to about1.8 and preferably between about 1 and 1.7.

The bath is operated at a temperature of from about 50 to about F. andpreferably from about 70 to about 110 F.

The chromate can be supplied by a Water soluble dichromate, e.g. sodiumdichromate, potassium dichrornate and ammonium dichromate.

Sodium permanganate and potassium permanganate are the preferred formsfor supplying the manganate.

Sulfuric acid (98 percent H 30 is a very satisfactory means forsupplying the small amount of sulfate ion utilized in the bath as wellas helping to maintain the pH of the solution Within the operable range.To illustrate, 1 and 3 milliliters of 98 percent H SO- per liter ofsolution are equivalent to about 0.2 and about 0.55 Weight percent SOrespectively in the treatment bath.

in practicing the invention, the article to be coated is degreased, ifnecessary, by a conventional degreasing method. The degreasing stepconventionally consists of immersing the article in or ubbing it with anaqueous alkaline solution, e.g., a Water solution of sodium carbonateand sodium hydroxide. The degreased magnesium is thereafter rinsed freeof residual alkali with Water and dried.

With magnesium articles, the degreasing step may be followed by anactivation step or they may be immersed without further preparation inthe novel coating solution of the present invention. Activation consistsof activating the surface of the magnesium, i.e., makin it a morereceptive site upon which to form the chromate coating. The activationmay be carried out by one of several Ways. Gne effective Way is toprepare a 0.5 percent by volume aqueous solution of 30 weight percentaqueous hydrofluosilicic acid or a 0.5 percent by volume aqueoussolution of 50 Weight percent hydrofluoric acid and to immerse themagnesium article in this solution at about room temperature or a littleabove, i.e. about 70 to 90 F. Immersion in the hydrofluosilicic acidsolution should be for about 0.5 minute and immersion in thehydrofluoric acid be from about 0.5 to about 2 minutes and preferablyfor about one minute.

If desired, for magnesium the degreasing and activating steps may becombined in a single step. Either of the two s utions, A and B set outbelow, will serve as a bath in which the magnesium may be immersed in acombined step. The bath temperature should be about to about F. and theduration of the immersion between about 1 and 5 minutes, but preferablyabout 2 minutes.

Solution A.50 grains of tetrasodium pyrophosphate (Na/ 0 and 20 grams ofsodium metaborate (NaBO -2H O) dissolved in sufficient water to make 1liter of solution at room temperature.

Solution B.70 grams of sodium itetraborate or borax (Na B G -lGI-E O),40 grams of tetrasodium pyrophospliate (Na P O- and 15 grams of sodiumfluoride (Na?) dissolved in suiiicient Water to make 1 liter of solutionat room temperature.

The activating step is completed by rinsing the magnesium article wellwith water.

After the degreasing step, or the degrea'sing and activating steps ifboth are used with magnesium, the article is treated by immersing fromabout 10 to about 90 seconds and preferably from about 15 to about 30seconds in the acidic aqueous ohrornate-manganate bath of the presentinvention.

After treatment in the chromate-manganate solution the article may bewater-rinsed and dried.

The chromate-manganate solution of the invention may 3 be used over along period of service by additions at intervals of the activeingredients therein.

The following examples will serve to illustrate further the presentinvention but are not meant to limit it thereto.

EXAMPLE 1 A 4 by 6 inch die-cast panel of A2913 magnesium alloy asdesignated by ASTM nomenclature (nominal composition 9 percent aluminum,0.15 manganese and 0.7 percent zinc) was degreased by inunersing forabout 3 minutes at 170 F. in a solution prepared by dissolving 40 gramsof tetrasodium pyrophosphate, '70 grams borax and 15 grams sodiumfluoride 'm sufiicient water to make a liter of solution. The so-treatedpanel was removed from the solution, rinsed with hot water for about 15seconds at 180 F. and then rinsed with cold water. The rinsed panel wasthereafter immersed for 15 seconds at about 75 F. in the novel aqueouschromate-manganate solution of the invention having a pH value of 1.15,contmning the following active constituents or ingredients:

Sodium dichromate (Na Cr O -2H O) (180 grams per liter) Potassiumpermanganate (KMnOQ grams per liter) Sulfuric acid (98 percent H 80 (2.5mol. per liter) The sotreated panel was removed from the solution,rinsed Well with cold water, followed by a hot Water rinse for secondsat 150 F. and force dried for 10 minutes at 225 F.

A very good uniform light brown adherent and nonpowdery coating wasproduced on the panel by this procedure. A number or" die-cast panelswere treated in the present solution and these were tested in highhumidity and exterior corrosion type test medium using panels havingbare untreated surfaces as controls. Other panels having the instantchromate-manganate conversion coating were painted using an automotivepaint system and these panels were also treated in high humidity andsalt spray corrosion tests. The results of these tests are shown inTable I.

Table I UNPAINTED Percent Surface Corrosion or Oxidation UnpaintedCondition Treatment 3 Months Industrial Exterior Exposure 100 Hrs. in95% Relative Humidity at 95 F.

%% Corrosion and No Oxidation. 5% Corrosion and 85% Oxidation.

3% Corrosion and No Oxidation. 10% Corrosion and 100% Oxidation.

Chromatc-Manganate- No treatment (control) 1 Paint system-1 coat of amodified alkyd yellow zinc chromate primer baked mins. at 250 F. plus 1coat of a modified alkyd dark gray low gloss metallic topcoat barred 25mins. at 250 F.

EXAMPLE 2 Panels of AZ3 1B magnesium alloy (nominal composition 3percent aluminum, .04 percent manganese and 1 percent zinc) wereprecleaned and given the chromatomanganate treatment following theprocedures and using solution concentrations as described in Example 1using a second immersion time in the chromate-manganate bath. A uniformbrown adherent coating was obtained by this procedure. The so treatedAZSlB magnesium alloy extruded panels were compared in high humidity,salt spray and industrial or. erior exposure corrosion type or" testingwith or without paint using as controls bare A2313 panels which have notbeen given the ohromatemanganae treatment. The results of these testsare shown in Table II.

1 Paint system-1 coat of a polyvinyl butyral zinc chromatc primer baked20 mins. at 300 F. plus 2 coats of Mil-E4729 alkyd white gloss enamelforce dried 2 hours at 160 F.

EXAMPLE 3 Example of the application of the chromate-manganate treatmentto Zinc base die castings.

Zinc alloy (ASTM AG40A alloyAl-4%, Cu- 0.25%, Mg-0.03%, balance Zn) diecastings were treated by the following procedure.

(1) Alkaline cleaned 3 mins. in 6% trisodium phosphate at 200 F.

(2) Cold water rinsed.

(3) Immersed 15 secs. in the chromate-manganate treatment using the samesolution as was used in the previous examples.

(4) Cold Water rinsed.

(5) Hot water rinsed 15 secs. at 150 F.

(6) Force dried 10 mins. at 225 F.

The above chromate-manganate treated zinc die castlugs and chromatetreated zinc die castings treated with a commercial chromate treatmentused for zinc die castings were painted with one coat of a modifiedalkyd zinc chromate primer baked for 25 mins. at 250 F. plus one coat ofan alkyd dark gray low gloss metallic topcoat baked 25 mins. at 250 F.or given a single coat of the alkyd topcoat. The castings were thenexposed to accelerated corrosion tests in percent relative humidity at95 F. to check paint blistering due to corrosion and/ or interior paintadhesion. The results of this test are shown in Table III which follows:

Commercial chromate treatment for zinc die castings based on sodiumdichromate and sulphuric acid.

In a manner similar to the foregoing examples, a chromate-manganatetreatment bath can be prepared utilizing about 245 grams KgClgO about8.5 grams NaMnO and about 3 cubic centimeters of 98 percent H SO perliter of solution. Also the bath can be prepared utilizing about 24grams (Ni-lQ Cr O about 1 gram KMnO and about 1 cubic centimeter of 98percent H 50 per liter of solution.

Various modifications can be made in the present invention withoutdeparting from the spirit or scope thereof for it is understood that Welimit ourselves only as defined in the appended claims.

We claim:

1. A composition for forming a protective surface coating on magnesiumarticles and zinc articles consisting essentially of an aqueous solutioncontaining as efiective constituents based upon the weight of thesolution between about 2.0 and 25.0 weight percent chromate ionexpressed as Na Cr O -2H O, about .05 to about 1 weight percent or"manganate ion expressed as KMnO and about 1 to 3 milliliters of sulfuricacid per liter of solution, said sulfuric acid containing about 98percent H 80 2. A composition for forming a protective surface coatingon magnesium articles and zinc articles consisting essentially of anaqueous solution containing as effective constituents based upon theweight of the solution between about 12 and 18 percent of chromate ionexpressed as Na Cr O -2H O, about 0.3 to about 0.6 percent manganate ionexpressed as KMnCq, and about 2 to 2.5 milliliters of 98 percent H 80conta ning sulfuric acid per liter of solution.

3. In a method of producing a coating on an article composed of at least85 percent magnesium, the step which consists of subjecting, at atemperature of from about 50 to about 140 F., the article, for a periodof between and 90 seconds, to the action of an aqueous solution having apH between 0.9 and 1.8, said solution containing as effectiveconstituents based upon the Weight of the solution between about 2 andpercent of chromate ion expressed as N21 Cr O -2H O, between about .05and 1 weight percent of manganate ion expressed as KMnO and about 0.2 toabout 0.55 percent sulfate ion expressed as 98 percent sulfuric acid.

4. In a method of producing a coating on an article composed of at least85 percent zinc, the step which consists of subjecting, at a temperatureof from about 50 to about 140 F. the article, for a period of between 10and 90 seconds, to the action of an aqueous solution having a pH between0.9 and 1.8, said solution containing as effective constituents basedupon the weight of the solution between about 2 and 25 percent ofchromate ion expressed as Na Cr O -2H O, between about .05 and 1 weightpercent of manganate ion expressed as KMnO; and about 0.2 to about 0.55percent sulfate ion, expressed as 98 percent sulfuric acid.

5. In a method of producing a coating on an article composed of at least85 percent magnesium, the step which consists of subjecting the article,for a period of time between about 15 and 30 seconds and at atemperature of from about to 110 F. to the action of an aqueous solutionof pH between about 1 to 1.7, said solution containing as effectiveconstituents, based upon the weight of the solution, between about 12and 1 8 percent of chromate ion expressed as Na Cr O -2H O, about 0.3 toabout 0.6 percent manganate ion expressed as KMnOL; and about 0.36 toabout 0.45 percent sulfate ion expressed as 98 percent sulfuric acid.

6. In a method of producing a coating on an article composed of at leastpercent zinc, the step which consists in subjecting the article, for aperiod of time bet-ween about 15 and 30 seconds and at a temperature offrom about 70 to F. to the action of an aqueous solution of pH betweenabout 1 to 1.7, said solution containing as effective constituents,based upon the weight of the solution, between about 12 and 18 percentof chromate ion expressed as Na Cr D 'ZH Q, about 0.3 to about 0.6percent manganate ion expressed as KMnO, and about 0.36 to about .45percent sulfate ion expressed as 98 percent sulfuric acid.

References Cited in the file of this patent UNITED STATES PATENTS2,352,076 Bushrod June 20, 1944 2,465,443 Gide Mar. 29, 1949 2,480,448Coates Aug. 30, 1949 2,681,873 Deniston June 22, 1954 FOREIGN PATENTS510,353 Great Britain July 24, 1939 OTHER REFERENCES Metallurgia,February 1951, pp. 53-61; p. 58, FIG. 8,

curve 4, relied on.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N06 3 l22457 F'ebruary'2L-3 1964 Herbert K. De Long et ale It is hereby cert entrequiring cor corrected below.

ified that error appears in the above numbered patrection and that thesaid Letters Patent should read as Column 3 line 25,, for "mol. read ml,

Signed and sealed this 27th day of October 1964 SEAL fittest:

ERNEST W; SWIDER Attesting Officer EDWARD J. BRENNER Commissioner ofPatents

3. IN A METHOD OF PRODUCING A COATING ON AN ARTICLE COMPOSED OF AT LEAST85 PERCENT MAGNESIUM, THE STEP WHICH CONSISTS OF SUBJECTING, AT ATEMPERATURE OF FROM ABOUT 50 TO ABOUT 140*F., THE ARTICLE, FOR A PERIODOF BETWEEN 10 AND 90 SECONDS, TO THE ACTION OF AN AQUEOUS SOLUTIONHAVING A PH BETWEEN 0.9 AND 1.8, SAID SOLUTION CONTAINING AS EFFECTIVECONSTITUENTS BASED UPON THE WEIGHT OF THE SOLUTION BETWEEN ABOUT 2 AND25 PERCENT OF CHROMATE ION EXPRESSED AS NA2CR2O7.2H2O, BETWEEN ABOUT .05AND 1 WEIGHT PERCENT OF MANGANATE ION EXPRESSED AS KMNO4 AND ABOUT 0.2TO ABOUT 0.55 PERCENT SULFATE ION EXPRESSED AS 98 PERCENT SULFURIC ACID.